A pressurized-water reactor power installation includes the vertical pressure vessel containing a core formed by vertical fuel assemblies supported in a core barrel forming between its outside and the inside of the pressure vessel, a descent space for pressurized-water coolant. At a level above the top of the core, the vessel has two or more coolant inlet connections and two or more outlet connections, the inlet connections communicating with the space between the core barrel and the vessel and descending through this descent space to the bottom of the vessel and rising upwardly through the inside of the core barrel via its opened bottom, the core barrel above the top level of the core communicating with the outlet connections. This permits the coolant to circulate upwardly through the core to cool individual vertical fuel rods mounted in clusters by the fuel assemblies.
The vessel's outlet connections connect with main coolant pipe line loops each having a hot leg which goes directly to the inlet of the primary header of a steam generator where the coolant circulates through the U-tube bundle heat-exchanger in the steam generator housing to which feed water is introduced for heating to steam, the steam leaving the steam generator for use as a power source. The steam generator primary header's outlet connects with the primary loop cold leg containing the coolant circulating main coolant pump and which goes back to the pressure vessel where it connects with the vessel's inlet connection. One such main coolant loop is provided for each steam generator included by the installation. The terms, hot leg and cold leg, are used because in the hot leg the coolant is carrying away the core heat to the steam generator where the coolant gives up the heat to the feed water, the cold leg carrying the coolant of reduced temperature back to the pressure vessel for recirculation through its core.
The core is within the lower portion of the pressure vessel and the core barrel extends upwardly to above the level of the vessel's coolant connections and contains above the core an upper guide and support structure containing interspaced control rod guide tubes for control rods which extend downwardly through the vessel's top or cover and via these guide tubes downwardly into the core below, the control rods being used to control the reactivity of the core by being raised and lowered via external control rod drive mechanisms above the outside of the vessel's cover. The top of the core barrel terminates below the inside of the vessel's cover and has a core barrel cover plate with openings through which the control rod guide tubes extend down through the guide tubes of the upper guide and support structure.
Thus, the core barrel defines a space between the top of the core and the bottom of the core barrel cover plate and which communicates with the hot leg connections which extend through the vessel's wall. This space is traversed only by the control rod guide tubes of the upper guide and support structure, these guide tubes are relatively widely interspaced horizontally from each other, and therefore, this space above the core within the core barrel, formed between and around the guide tubes, is relatively open and of large area.
Although unlikely, it is possible for one of the coolant loops to suffer a break while the reactor is in operation. In that event the coolant circulation through the core stops. Reactor installations necessarily have a reactor protective system which is triggered should such an accident occur, all of the control rods being then dropped to terminate the reactivity of the core. However, the residual heat of the core and the decay heat of its fuel cause the temperature of the core to rapidly rise. Therefore, a reactor installation necessarily incudes an emergency core cooling system for use in the event of such a loss-of-coolant accident.
Such a system includes an accumulator containing a supply of emergency cooling water under gas pressure in the accumulator tank and which is intended to be adequate, if promptly and properly introduced to the core vessel, to keep the core temperature under control until other remedial measures may be taken.
Such a system is also activated by the reactor protective system, when required.
With the core temperature rapidly rising, when the coolant circulation stops, the introduction to the vessel of the emergency cooling water involves problems. With the main loop suffering a break, the pressure on the water coolant drops within the pressure vessel so that coolant remaining there converts to steam which drives out of the vessel and escapes via the break. Therefore, one of the problems involved by the introduction of the emergency cooling water to the vessel is the production of steam by contact of the water with the overheating core, this possibly filling the vessel, particularly its upper portion, with steam under high pressure. The emergency cooling water must be forced into the vessel against this pressure and this may require a force beyond the capacity of the accumulator gas pressure. If a pump is relied on to feed the emergency cooling water supply into the vessel, this pump must be very large and, therefore, expensive to provide the necessary pressure on the emergency cooling water and which must, of course, be higher than the steam pressure created within the vessel.
Normally the emergency cooling water is introduced to the vessel via either or both of the hot and cold legs of the main coolant loop, although it may be introduced via connections formed for this purpose through the vessel's wall. It is, of course, necessary for the emergency cooling water to get to the bottom of the vessel and fill the latter upwardly to above the top level of the core and to the vessel's coolant connections, and this should be done as quickly as possible.